Spring MVC Как вытащить из модели атрибут поменять его формат и вставить обратно

   @RequestMapping(value = "/", method = RequestMethod.GET)
    public String listStudents(Model model)
    {
        List<Student> students = studentDAO.getAllStudents();
        model.addAttribute("students", students);

private String imgBase64;

@RequestMapping(value = "/", method = RequestMethod.GET)
    public String listStudents(Model model)
    {
        List<Student> students = studentDAO.getAllStudents();
 
        for (Student student : students) {
            StudentDto studentDto = new StudentDto();
            Long id = student.getId();
            studentDto.setId(id);
            String firstName = student.getFirstName();
            studentDto.setFirstName(firstName);
            String lastName = student.getLastName();
            studentDto.setLastName(lastName);
            Date dateOfBirth = student.getDateOfBirth();
            studentDto.setDateOfBirth(dateOfBirth);
            String gender = student.getGender();
            studentDto.setGender(gender);
            Address address = student.getAddress();
            studentDto.setAddress(address);
            byte[] img = student.getImg();
            studentDto.setImg(img);
        }
 
        List<StudentDto> studentDtos = new ArrayList<>();
     .....

        List<Student> students = studentDAO.getAllStudents();
        List<StudentDto> studentDtos = new ArrayList<>();  //объявить перед циклом
 
        for (Student student : students) {
            StudentDto studentDto = new StudentDto();
 
            Long id = student.getId();
            studentDto.setId(id);
            String firstName = student.getFirstName();
            studentDto.setFirstName(firstName);
            String lastName = student.getLastName();
            studentDto.setLastName(lastName);
            Date dateOfBirth = student.getDateOfBirth();
            studentDto.setDateOfBirth(dateOfBirth);
            String gender = student.getGender();
            studentDto.setGender(gender);
            Address address = student.getAddress();
            studentDto.setAddress(address);
            byte[] img = student.getImg();
            studentDto.setImg(img);
 
            studentDtos.add(studentDto);      //добавить в коллекцию
        }

Ранова-то Вы за Спринг взялись.

model.addAttribute("students", studentDtos);

          ........
           byte[] img = student.getImg();
            studentDto.setImgBase64(new String (Base64.getEncoder().encode(img)));
 
            studentDtos.add(studentDto);
        }

Message Request processing failed; nested exception is java.lang.NullPointerException
Description The server encountered an unexpected condition that prevented it from fulfilling the request.
Exception
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException

studentDto.setImgBase64(new String (Base64.getEncoder().encode(img)));

....
byte[] img = student.getImg();
            String imageBase64Str = "blblblblblblblb";
            studentDto.setImgBase64(imageBase64Str);
.....

byte[] img = student.getImg();

....
@RequestMapping(value = "/addStudent", method = RequestMethod.POST)
    public String addStudent(@ModelAttribute("student") Student student) {
        studentDAO.addStudent(student);
        return "redirect:/";
    }
....